Method for making a honeycomb structure comprising coating a honeycomb body with cement skin layers

ABSTRACT

Disclosed is a ceramic honeycomb structure comprising a honeycomb body and a multilayered outer layer formed of a thick core layer applied and rapidly dried and a thin clad layer dried more gently to form a crack free dual skin layer. The core layer may have properties that are closer to those of the ceramic honeycomb body in service than the clad layer that may provide a tough outer shell to withstand handling and assembly.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims the benefit of U.S.patent application Ser. No. 13/688,891, filed Nov. 29, 2012, which ishereby incorporated by reference for all purposes as if fully set forthherein.

BACKGROUND

Field

Exemplary embodiments of the present disclosure relate generally tohoneycomb structures, and to ceramic honeycomb particulate filters andsubstrates comprising a multi-layer outer skin layer exhibiting improvedthermal expansion compatibility with the underlying honeycomb body,enhanced iso-static strength of the honeycomb structure, and improvedmanufacturing process times.

Technical Background

Particulate filters and substrates with large diameters may be difficultto manufacture to the tight dimensional requirements set by originalequipment manufacturers (OEMs) and the supply chain due to unpredictabledrying and firing shrinkage. Consequently, a cold set ceramic cement maybe used to form an exterior skin of a honeycomb monolith includinghoneycomb monoliths formed by honeycomb segments. The cold set ceramiccement is mixed and applied to a fired, contoured or segmented substrateand the wet skin is afterward allowed to dry either under ambientconditions or by convective or microwave drying at elevatedtemperatures. The dried part is then ready to receive a catalyst coatingand any further downstream processing if required.

Present skin designs embody conflicting physical property requirementsbetween the final application and the intermediate processing andhandling. In service, the material must have a low thermal expansion andhigh thermal shock resistance (low elastic modulus) to endure severethermal gradients. To achieve these properties, high porosity and weakbonding are desired for maximum skin flexibility. To withstand shipping,handling and/or processing (including extreme pH exposure duringcatalyzation), high strength and chip/abrasion resistance are desired.To achieve these properties, low porosity and strong bonding between theskin and the ceramic body are sought for maximum skin strength.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention asclaimed and therefore it may contain information that does not form anypart of the prior art nor what the prior art may suggest to a person ofordinary skill in the art.

SUMMARY

An exemplary embodiment provides a honeycomb structure comprising ahoneycomb body including a plurality of cells extending axially betweenfirst and second opposing end faces of the honeycomb body, the cellscomprising intersecting porous walls, and a first cement mixture layerdeposited over a periphery of the honeycomb body. The first cementmixture layer may comprise a green cement mixture in that the cementmixture may be dried but not calcined, or the first cement mixture layermay be a calcined cement mixture. The honeycomb structure includes asecond cement mixture layer deposited over the first cement mixturelayer. The second cement mixture layer may comprise a green cementmixture, or the second cement mixture layer may be a calcined cementmixture.

An exemplary embodiment also discloses a method of making a honeycombstructure including coating the periphery of a honeycomb body with afirst cement mixture, drying the first cement mixture on the peripheryof the honeycomb body to form a first skin layer, coating the first skinlayer with a second cement mixture, drying the second cement mixture onthe periphery of the honeycomb body to form a second skin layer.

An exemplary embodiment also discloses a system for applying a dual skinlayer to a honeycomb body including a first cement mixture applicatorunit configured to coat a periphery of the honeycomb body with a firstcement mixture, a drying unit configured to dry the first cement mixtureto form a first skin layer on the periphery of the honeycomb body, asecond cement mixture applicator unit configured to coat the first skinlayer with a second cement mixture to be a second skin layer on theperiphery of the honeycomb body when dried.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or recognized by practicing the invention.

It is to be understood that both the foregoing general description andthe following detailed description present exemplary embodiments of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the disclosure, and are incorporated into andconstitute a part of this specification. The drawings illustrate variousexemplary embodiments of the claimed invention, and together with thedescription serve to explain the principles and operations of theclaimed invention.

FIG. 1 is a perspective view of a honeycomb structure according to anexemplary embodiment of the present disclosure.

FIG. 2 is an end view of the honeycomb structure of FIG. 1.

FIG. 3 is a schematic illustrating a model for drying by internaldiffusion and surface evaporation.

FIG. 4A is a graph illustrating gradual water content gradients for themodel shown in FIG. 3 under slow drying conditions such as overnight,and FIG. 4B is a graph illustrating steep water content gradients forthe model shown in FIG. 3 under rapid drying conditions.

FIGS. 5A, 5B, and 5C are graphs illustrating water content gradients forthe model shown in FIG. 3 in the presence of a non-porous (FIG. 5A),medium porous (FIG. 5B) and highly porous (FIG. 5C) honeycomb body thatabsorbs moisture from the skin, hence relaxing the moisture gradient.

FIG. 6 is a flow diagram illustrating an exemplary process to make adouble-skinned product according to exemplary embodiments of thedisclosure.

FIG. 7 is a schematic of a honeycomb body multi-layer skinning systemaccording to an exemplary embodiment of the disclosure.

FIG. 8 is a perspective view of a honeycomb structure according to anexemplary embodiment of the present disclosure.

FIG. 9A is a photograph showing an example core skin on a 2 in diameterhoneycomb body according to an exemplary embodiment. Defects resultingfrom fast drying are visible. FIG. 9B is a photograph showing an exampleclad skin on a 2 in diameter honeycomb body having a core skin as shownin FIG. 9A according to an exemplary embodiment. Defects are eliminatedthrough the second layer of skin. FIG. 9C is a photograph of thecorresponding ends of the examples shown in FIGS. 9A and 9B.

FIG. 10A is a photograph showing an example core skin on a 2 in diameterhoneycomb body according to an exemplary embodiment. Defects resultingfrom fast drying are visible. FIG. 10B is a photograph showing anexample clad skin on a 2 in diameter honeycomb body having a core skinas shown in FIG. 10A according to an exemplary embodiment. Defects areeliminated through the second layer of skin. FIG. 10C is a photograph ofthe corresponding end of the example shown in FIG. 10B.

FIG. 11A is a photograph showing a side view of an example dual layerskin on a 2 in diameter honeycomb body according to an exemplaryembodiment. FIG. 11B is a photograph showing an end view of the duallayer skin on the 2 in diameter honeycomb body shown in FIG. 11A. Thedual layer skin can be crack-free even when off-center according toexemplary embodiments as demonstrated by the example shown in FIGS. 11Aand 11B.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of thedisclosure, examples of which are illustrated in the accompanyingdrawings. Whenever possible, the same reference numeral will be usedthroughout the drawings to refer to the same or like parts. It will beunderstood that when an element or layer is referred to as being “on” or“connected to” another element or layer, it can be directly on ordirectly connected to the other element or layer, or interveningelements or layers may be present. In contrast, when an element or layeris referred to as being “directly on” or “directly connected to” anotherelement or layer, there are no intervening elements or layers present.It will be understood that for the purposes of this disclosure, “atleast one of X, Y, and Z” can be construed as X only, Y only, Z only, orany combination of two or more items X, Y, and Z (e.g., XYZ, XYY, YZ,ZZ).

As used herein, a green material is an unfired material comprising amixture of inorganic and/or organic materials. The green material mayinclude various inorganic filler materials, inorganic and/or organicbinder materials, and liquid vehicle. The green material may be dried toremove fluid content (e.g. water). Drying is often accomplished byallowing a part to sit exposed to the ambient atmosphere overnight,however, hot air, forced air, microwave, radio frequency (RF) orinfrared radiation (IR) may be used to augment drying. The drying may beaccomplished in humidity controlled air. Green material may includecold-set cements.

As used herein, calcination refers to heating of a green material to atemperature less than 1000° C. for a period sufficient to burn outorganic materials contained within the material, for example, 600° C.for about 3 hours.

As used herein, a “super addition” refers to a weight percent of acomponent, such as, for example, an organic binder, liquid vehicle,additive or pore former, based upon and relative to 100 weight percentof the inorganic components of the mixture.

Shown in FIGS. 1 and 2 is an exemplary honeycomb structure 10 accordingto one embodiment comprising a honeycomb body 12. The honeycomb body 12has a longitudinal axis 14 and a length L, and comprises a plurality ofintersecting porous walls 16 that form mutually adjoining cells orchannels 18 extending axially between opposing end faces 20, 22. Celldensity can be between 100 and 900 cells per square inch. Typical cellwall thicknesses can range from about 0.025 mm to about 1.5 mm. As usedherein, the term “honeycomb” is intended to include a generallyhoneycomb structure but is not strictly limited to a square structure.For example, hexagonal, octagonal, triangular, rectangular or any othersuitable shape may be used. Typical pore sizes contained within theporous walls can be from 0.1 μm to about 100 μm, with cell wall porositybetween about 15% and 75%, for example, between about 25% and 60%.

Honeycomb body 12 may be formed from a ceramic material, such ascordierite (2MgO-2Al₂O₃-5SiO₂). However, limited substitution of otherconstituents such as Fe (iron), Co (cobalt), Ni (nickel) and Mn(manganese) for Mg (magnesium), Ga (gallium) for Al (aluminum) and Ge(germanium) for silicon is acceptable. Also, the cordierite phase mayinclude alkali metals, alkaline earth metals or rare earth metals.Honeycomb body 12 may in other cases be made of other ceramic materials,such as silicon carbide, silicon nitride, aluminum titanate, aluminaand/or mullite, or combinations thereof.

The honeycomb body can be formed according to any conventional processsuitable for forming honeycomb monolithic bodies. For example, aplasticized ceramic forming batch composition can be shaped into a greenbody by any known conventional ceramic forming process, such asextrusion, injection molding, slip casting, centrifugal casting,pressure casting, dry pressing and the like. Typically, honeycombstructures are formed by an extrusion process where a ceramic materialis extruded into a green form before the green form is fired to form thefinal ceramic structure. In an exemplary embodiment, the extrusion canbe performed using a hydraulic ram extrusion press, a two stagede-airing single auger extruder or a twin screw mixer with a dieassembly attached to the discharge end. The extruded material can be cutto create honeycomb structures such as filter bodies shaped and sized tomeet the needs of engine manufacturers. The extruded material can behoneycomb segments connected or bonded together to form the honeycombstructures. These extruded green bodies can be any size or shape.

Generally, as a ceramic honeycomb structure is extruded, a solidexternal surface is provided along the length of the structure. Undercertain circumstances, however, it may become necessary to remove theexternal surface. For example, a green extruded honeycomb structure maybe shaped to a desired shape and size by removing the extruded externalsurface. Alternatively, the green honeycomb structure may be fired andthen ground to the desired shape and size by removing the externalextruded surface and any portion of the porous wall structure necessaryto attain the desired shape and size. Shaping can be accomplished by anymeans known in the art, including cutting, sanding or grinding away theouter extruded surface of the honeycomb structure to achieve the desiredshape and size.

Likewise, honeycomb segments may be shaped to a desired shape and sizeby removing the extruded external surface before integrating to thehoneycomb structure. Alternatively, the honeycomb segments may beintegrated to form a honeycomb structure and the formed honeycombstructure shaped to the desired shape and size.

Once the desired shape and size has been attained, a cement material canbe applied to an outer periphery of the sized body to form a newexternal surface, or skin, on the body. Typically, the ends of thehoneycomb body are not covered with the cement, although certainpassages may be plugged if desired. Once the cement composition has beenapplied to the honeycomb structure, the cement composition can be driedand/or calcined. In some embodiments a cold-set cement composition maybe applied to the honeycomb structure. In some embodiments, thehoneycomb body over which the cement is applied comprises fired ceramicmaterial. In other embodiments, the honeycomb body comprises a greenbody or a calcined body. In some cases, final firing of the calcinedhoneycomb structure can take place during the catalyzation process.

Once the cement material has been applied to the honeycomb structure ina manner as described herein, the cement material can be optionallydried and/or fired. The optional drying step can comprise first heatingthe cement material in a humidity controlled atmosphere at a temperatureand for a period of time sufficient to at least substantially remove anyliquid vehicle that may be present in the cement material. As usedherein, at least substantially removing any liquid vehicle includes theremoval of at least 95%, at least 98%, at least 99%, or even at least99.9% of the liquid vehicle present in the cement material prior tofiring. Exemplary and non-limiting drying conditions suitable forremoving the liquid vehicle include heating the cement material at atemperature of at least 50° C., at least 60° C., at least 70° C., atleast 80° C., at least 90° C., at least 100° C., at least 110° C., atleast 120° C., at least 130° C., at least 140° C., at least 150° C., atleast 160° C., at least 170° C., at least 180° C., at least 190° C., oreven at least 200° C. In one embodiment, the conditions effective to atleast substantially remove the liquid vehicle comprise heating thecement material in a humidity controlled atmosphere, such as air, at atemperature in the range of from 60° C. to 120° C. Further, the heatingcan be provided by any conventionally known method, including forexample, hot air drying, RF and/or microwave drying in a humiditycontrolled atmosphere.

The optional firing step can include conditions suitable for convertingthe cement material to a primary crystalline phase ceramic compositioninclude heating the honeycomb with applied cement material to a peaktemperature of greater than 800° C., 900° C., and even greater than1000° C. A ramp rate of about 120° C./hr during heating may be used,followed by a hold at the peak temperature for a temperature of about 3hours, followed by cooling at about 240° C./hr.

Cement material disclosed herein can include those that set at atemperature of less than 200° C., such as a temperature of less than100° C., and further such as a temperature of less than 50° C.,including cement material that can be used in skinning processesemploying “cold set” skins. In cold set skinning, only drying of theskinning mixture is required to form a seal of the channel walls of thehoneycombs. When a cold set skinning process is employed, heating of theskinned honeycombs to temperatures in the 35-110° C. range can be usefulto accelerate drying. In some cold set skinning processes, it isanticipated that final skin consolidation, including the removal ofresidual temporary binder bi-products and strengthening of the seals,can occur in the course of subsequent processing steps (e.g., in thecourse of catalyzation or canning) or during first use (e.g., in anexhaust system).

For example, exemplary compositions in which cold set skinning may beemployed include those comprising a refractory filler that comprises atleast one inorganic powder, such as at least one of aluminum titanate,cordierite, fused silica, mullite, and alumina, the inorganic powderhaving a bimodal or mono sized median particle size (D₅₀) of from 15 to50 microns, such as from 30 to 40 microns for mono sized andadditionally a median particle size in a range from about 150 microns toabout 300 microns, such as from about 150 microns to about 250 micronsfor the second particle size in bimodal size compositions, and a gelledinorganic binder, such as gelled colloidal silica. At least one gellingagent, such as at least one of hydrochloric acid, sulfuric acid, nitricacid, citric acid, and acetic acid, ammonium hydroxide, sodiumhydroxide, and triethanol amine (hereinafter “TEA”) may be added eitherbefore (e.g., as a pre-mix with the gelled inorganic binder) or duringbatching in order to gel the inorganic binder. Alternatively anon-gelled composition may be used. Such compositions can provide skinsthat set in a porous ceramic honeycomb body (and be thereby permanentlysealed to the channel walls) at a temperature of less than 200° C., suchas less than 100° C., and further such as less than 50° C., includingabout 25° C. Further non-limiting exemplary embodiments of cementcompositions used for skinning are discussed below.

Various methods may be employed to apply a layer of cement to thehoneycomb body 12. For example, a dispensing device (not shown) can beused to apply an appropriate amount of cement mixture to the externalsurface of the honeycomb body 12. Methods of applying a skin material(e.g. cement) may include an axial application method, a doctor bladesqueegee application method, a spray casting method, a tape castingmethod, a spin coating method, and the like. For example, U.S. patentapplication Ser. No. 13/463,125, the content of which is incorporatedherein by reference in its entirety, describes various methods ofapplying a cement skin to a honeycomb body. For another example, U.S.patent application Ser. No. 12/231,140, the content of which isincorporated herein by reference in its entirety, also describes variousmethods of applying a cement skin to a honeycomb body.

Cement skins may crack when dried under humidity controlled conditions,for example, at ambient conditions, in a high temperature hot air drier,in a microwave heater, in a radio frequency (RF) heater, or in aninfrared (IR) heater. Skin thickness variability may also contribute tothe development of skin drying cracks. Longer times and lowertemperature heating may be employed to avoid drying cracks, but at theexpense of process through put efficiencies. Cracks on the skin may bemanually detected after the drying process, and then manually fixed withskin cement. This may add further cost in terms of labor and time forinspecting and fixing of drying cracks. To reduce cracking during dryinga low evaporation rate may be required. A low evaporation rate at theskin surface makes the drying process slow adding to manufacturing timeand cost.

FIG. 3 illustrates a model for calculating drying of a cement mixture ona periphery of a honeycomb body to form a skin layer. The modelcalculates drying by internal diffusion and surface evaporation. Thehoneycomb body 12 forms an interface 24 with the layer of cement mixture28 applied to the outer periphery of the honeycomb body 12. Theinterface 24 of cement mixture layer 28 and honeycomb body 12 is treatedas insulation in the model. A cement mixture layer surface and airinterface 32 is treated as convective mass transfer in the model.

FIG. 3 shows the schematic of the internal diffusion and surfaceevaporation based model. The interface 24 of the cement mixture layer 28(skin) and honeycomb body 12 (filter or substrate) is treated as aninsulated layer for low porosity filters as water movement through a lowporosity honeycomb body 12 is limited. In contrast, for high porosityfilters, a separate mass transfer coefficient is provided at thehoneycomb body 12 interface 24. The cement mixture layer 28 to airinterface 32 loses moisture due to evaporation. The evaporation rate isa function of temperature, humidity and velocity of air.

FIG. 4A is a graph illustrating gradual water content gradients for themodel shown in FIG. 3 under slow drying conditions such as overnight(natural convection), and FIG. 4B is a graph illustrating steep watercontent gradients for the model shown in FIG. 3 under rapid dryingconditions. In FIGS. 4A and 4B the gradients decrease as dryingprogresses as indicated by the drying time arrows. The steep gradientsshown in FIG. 4B are formed in the case of forced convection drying(blowing hot air) because diffusion inside the skin cannot keep pacewith the high evaporation rate at the air interface 32 surface of thecement mixture layer 28. In contrast, drying overnight shown in FIG. 4Aleads to gentle gradients because the evaporation rate and the internaldiffusion rates are similar.

FIGS. 5A, 5B, and 5C are graphs illustrating water content gradients forthe model shown in FIG. 3. FIG. 5A is a graph illustrating water contentgradients for the model shown in FIG. 3 in the presence of a non-poroushoneycomb body that does not absorb much moisture from the skin. FIG. 5Bis a graph illustrating water content gradients for the model shown inFIG. 3 in the presence of a medium porous honeycomb body that absorbssome moisture from the skin, and FIG. 5C illustrates water contentgradients in the presence of a highly porous honeycomb body that absorbsmoisture from the skin, hence relaxing the moisture gradient. Theresults from the models were combined with experimental basedobservations of drying of a cement mixture on a periphery of a honeycombbody to form a skin layer as described below.

While the inventors observed that thin skins crack less than thickskins, thin skins are difficult to obtain consistently because ofseveral factors. For instance, centering the honeycomb body on arotation plate for coating with the cement mixture may be difficult, thefilter body may have uneven contouring before coating, or a combinationof these may cause variability in the thickness of the coating. Skinthickness variability generally makes the skin more susceptible todeveloping drying cracks.

Skin on top of a porous substrate/filter is less likely to crack than askin on top of a non-porous (low porosity) substrate. Drying crack-freeskins on low porosity substrates often presents a challenge.

Thin skins or fibrous skins are less prone to cracks during drying, butthey are fragile and prone to cracks during handling or transportation.

The inventors discovered that a honeycomb body coated with a firstcement mixture layer to form a first skin and a second cement mixturelayer to form a second skin on top of the first skin gave a surprisingcrack free dual layer outer cement skin in a short period of time.Referring back to FIG. 1, the first cement mixture layer forms a coreskin 36 and the second cement mixture layer forms a clad skin 40. Thecore skin 36 and the clad skin 40 may be of the same composition. Also,the core skin 36 and the clad skin 40 may be of different compositionsfrom each other. The first cement mixture layer may be applied andrapidly dried in a process that uses high temperature hot air, forcedair, microwave, RF, IR radiation, or the like. The rapid drying processmay cause cracks in the core skin layer, but those cracks are removedwhen the second cement mixture is formed into the skin clad layer overthe skin core layer. The thickness of the clad skin layer may be lessthan the thickness of the core skin layer. The drying process of theclad skin layer may be a slow drying process such as low temperature hotair drying, low power microwave, low power RF, low power IR radiation,and the like. The thinner layer and slower drying of the clad layer wasdiscovered to lead to a crack free dual layer outer cement skin.

To achieve a crack free dual layer outer cement skin that withstands inservice conditions the inner skin layer (core skin) exhibits a lowthermal expansion and high thermal shock resistance (low elasticmodulus) to endure severe thermal gradients. To achieve theseproperties, high porosity and weak bonding may be achieved for skinflexibility. The core skin may have a coefficient of thermal expansion(CTE) that substantially matches the honeycomb body.

The outer skin layer (clad skin) may be formed to withstand shipping,handling and/or processing (including extreme pH exposure duringcatalyzation). To achieve a crack free dual layer outer cement skin thatwithstands shipping, handling and processing, high strength andchip/abrasion resistance are desired. To achieve these properties, lowporosity and strong bonding between the clad skin and the core skin onthe ceramic body are sought for maximum clad skin strength.

Skin compositions are described in U.S. Provisional Patent ApplicationNo. 61/602,883 and U.S. patent application Ser. No. 13/302,262, thecontents of which are incorporated herein by reference in theirentirety. According to exemplary embodiments the skin composition may bea single glass powder composition including a cement comprising a glasspowder as a low thermal expansion filler material, a binder and asolvent or vehicle for carrying the solid constituents of theglass-based cement. The glass of the glass powder filler material may bean amorphous fused silica (SiO₂), ground cordierite, AT grog, or silicasoot. The glass powder filler material can have a median particle size(D50) between 10 and 20 μm, with a minimum particle size between 7 μmand 75 μm and a maximum particle size between 50 μm and 70 μm. Particlesize determined as a mass-based equivalent spherical diameter. The glasspowder filler material may comprise, for example, from 60-80 wt. % ofthe total inorganic components of the cement. Suitable silica powderfiller materials are available, for example, under the trade nameTeco-Sil, available from CE Minerals of Tennessee Electro MineralsIncorporated, Tennessee, USA. All particle size measurements herein weremade with a Microtrac Inc. particle size analyzer, unless otherwiseindicated.

According to exemplary embodiments the skin composition may include anamorphous glass-based cement, the cement formed from a dual glass powdercomposition comprising a first (fine) glass powder as a low thermalexpansion filler material, a second (coarse) glass powder as a lowthermal expansion filler material, a binder and a solvent or vehicle forcarrying the solid constituents of the glass-based cement. The glassesof both the first glass powder filler material and the second glasspowder filler material may be amorphous fused silica having particlesizes greater than about 1 μm. The distribution of glass powder fillermaterial particle size is preferably multimodal in that a distributionof the glass powder filler material with particle sizes greater thanabout 1 μm exhibits multiple modes (local maximums) of particle sizes.In one embodiment, the amorphous glass-based cement comprises a bimodalparticle size distribution of amorphous glass particles with a particlesize greater than about 1 μm. The glass based cement may include a firstglass powder filler material wherein a median (D50) particle size of thefirst glass powder filler material is preferably in a range from about10 to about 50 μm, from about 15 μm to about 50 μm, from about 20 μm toabout 45 μm or from about 30 μm to about 45 μm, with a D10 in a rangefrom about 1 μm to about 10 μm and D90 in a range from about 25 μm toabout 125 μm. A median (D50) particle size of the second glass powderfiller material is preferably in a range from about 150 μm to about 300μm, in a range from about 150 μm to about 250 μm, in a range from about170 μm to about 230 μm, in a range from about 180 μm to about 220 μm,with D10 in a range from about 100 μm to about 150 μm, and D90 in arange from about 250 μm to about 350 μm. Particle sizes are determinedas a mass-based equivalent spherical diameter. As used herein, the termD50 represents the median of the distribution of particle sizes, D10represents the particle size in microns for which 10% of thedistribution are smaller than the particle size, and D90 represents theparticle size in microns for which 90% of the distribution are smallerthan the particle size. The dual glass based cement may contain, forexample, an amount of the first glass powder filler material in a rangefrom about 20 to about 60 wt. % of the total weight of the inorganicsolid components of the cement, in a range from about 25 wt. % to about50 wt. %, in a range from about 25 wt. % to about 40 wt. %, or in arange from about 25 wt. % to about 35 wt. %. The glass based cement maycontain, for example, an amount of the second glass powder fillermaterial in a range from about 10 wt. % to about 40 wt. % of the totalweight of the inorganic solid components of the cement, in a range fromabout 15 wt. % to about 40 wt. %, in a range from about 20 wt. % toabout 35 wt. %.

In one exemplary embodiment, D50 of the first glass powder fillermaterial may be in a range from about 34 μm to about 40 μm, and a medianparticle size of the second glass powder filler material is in a rangefrom about 190 μm to about 280 μm. In one example, the first glasspowder filler material has a D10 of about 6.0 μm, a D50 of about 34.9 μmand a D90 of about 99 μm. In another example, the first glass powderfiller material has a D10 of about 6.7 μm, a D50 of about 39.8 μm, and aD90 of about 110.9 μm. In still another example, the first glass powderhas a D10 of about 2.7 μm, a D50 of about 13.8 μm and a D90 of about37.8 μm, and as yet another example, the first glass powder fillermaterial has a D10 of about 2.8 μm, a D50 of about 17.2 μm and a D90 ofabout 47.9 μm.

The ratio of the second glass powder filler material to the first glasspowder filler material may be in a range from about 1:4 to about 1:1,such as about 1:3.5 to about 1:1, from about 1:3 to about 1:1, fromabout 1:2.5 to about 1:1, from about 1.2 to about 1:1 or from about1:1.5 to about 1:1. In an exemplary embodiment, the ratio of the secondglass powder filler material to the first glass powder filler materialis 1:1.

To provide the cement compositions of the present disclosure, theinorganic powders comprising any of the above inorganic powders and anyoptional inorganic additive components can be mixed together with asuitable organic and/or inorganic binder material. The organic bindermaterial may comprise one or more organic materials, such as a celluloseether, methylcellulose, ethylcellulose, polyvinyl alcohol, polyethyleneoxide and the like, or in some embodiments a gum-like material such asActigum®, xanthan gum or latex. For example, A4 Methocel is a suitableorganic binder. Methocel A4 is a water-soluble methyl cellulose polymerbinder available from Dow Chemical. A suitable inorganic binder maycomprise colloidal silica or alumina comprising nanometer-scale silicaor alumina particles suspended in a suitable liquid, such as water. Theinorganic binder material is preferably present in the cementcomposition in an amount less than about 10% of the total weight ofinorganic solids present in the cement, and in some exemplaryembodiments inorganic binders are present in an amount equal to or lessthan about 5 wt. %, and in certain other exemplary embodiments in arange from about 2 wt. % to about 4 wt. % taking into account the fluidportion of the organic binder (wherein the weight contribution of thefluid portion is removed). A suitable colloidal silica binder materialis Ludox HS40 produced by W.R. Grace. Typical colloidal binder materialsmay comprise approximately 40% by weight solid material as a suspensionin a deionized water vehicle.

In some exemplary embodiments, the single and dual glass powder cementsdescribed supra may also include an inorganic fibrous reinforcingmaterial. For example, aluminosilicate fibers may be added to the cementmixture to strengthen the honeycomb structure after application of theskin. For example, the cement may include an inorganic fibrous materialfrom about 25 to about 50 wt. % of the total weight of the inorganicsolid components of the cement, from about 30 to about 50 wt. %, and insome embodiments from about 35 to about 45 wt. % of the total weight ofthe inorganic solid components of the cement. In certain otherembodiments, fibrous inorganic reinforcing materials may be present inan amount from about 36 wt. % to about 43 wt. % as a percentage of thetotal weight of the inorganic solids of the cement composition. Asuitable inorganic fibrous reinforcing material is Fiberfrax QF 180,available from Unifrax, however, any high aspect ratio refractoryparticulate could be used.

Typically, the preferred liquid vehicle or solvent for providing aflowable or paste-like consistency has included water, such as deionized(DI) water, although other materials may be used. The liquid vehiclecontent may be present as a super addition in an amount equal to or lessthan about 30 wt. % of the inorganic components of the cement mixture,preferably in a range from about 10 wt. % to about 25 wt. % of theinorganic components of the cement mixture. However, the liquid vehicleis typically adjusted to obtain a viscosity suitable to make the cementeasy to apply.

In some embodiments the cement may optionally further contain organicmodifiers, such as adhesion promoters for enhancing adhesion between thecement and the honeycomb body. For example, Michem 4983 has been foundsuitable for this purpose.

In accordance with exemplary embodiments disclosed herein, the averagecoefficient of thermal expansion (CTE) of the calcined cement from 600°C. on cooling to room temperature is equal to or less than about15×10⁻⁷/° C., preferably equal to or less than about 12×10⁻⁷/° C.,preferably equal to or less than about 10×10⁻⁷/° C., preferably equal toor less than about 8×10⁻⁷/° C. As used herein room temperature meansabout 23° C. Preferably, a coefficient of thermal expansion of thecalcined cement mixture is equal to a thermal expansion coefficient ofthe honeycomb body. In certain embodiments CTE can be in a range fromabout 7×10⁻⁷/° C. to about 10×10⁻⁷/° C.

In accordance with embodiments disclosed herein, the modulus of rupture(MOR) of uncalcined cement is in a range from about 20 kg/cm² to about60 kg/cm², preferably in a range from about 20 kg/cm² to about 50kg/cm², preferably in a range from about 20 kg/cm² to about 40 kg/cm²,preferably in a range from about 20 kg/cm² to about 35 kg/cm². Modulusof rupture of uncalcined cement in some embodiments is between 22 kg/cm²and 35 kg/cm². In certain embodiments, the uncalcined MOR is in a rangefrom about 24 kg/cm² to about 30 kg/cm².

In accordance with embodiments disclosed herein, the modulus of rupture(MOR) of calcined cement is in a range from about 14 kg/cm² to about 45kg/cm², preferably in a range from about 14 kg/cm² to about 40 kg/cm²,preferably in a range from about 14 kg/cm² to about 35 kg/cm²,preferably in a range from about 14 kg/cm² to about 30 kg/cm². Modulusof rupture of calcined cement in some embodiments is between 14 kg/cm²and 25 kg/cm². In certain embodiments, the calcined MOR is in a rangefrom about 14 kg/cm² to about 20 kg/cm².

Porosity of the calcined cement skin is in a range from about 30% toabout 60%, preferably in a range from about 35% to about 60%, andpreferably in a range from about 35% to about 50%. In some embodiments,porosity can be in a range from about 35% to about 48%.

Cement compositions described herein can exhibit viscosities well suitedfor forming an external skin over a honeycomb body. For example,compositions according to the embodiments herein can have an infiniteshear viscosity equal to or less than about 12 Pascal-seconds (Pa·s),equal to or less than about 5 Pa·s, or equal to or less than about 4Pa·s. For a shear rate of 10 s⁻¹, the shear viscosity is preferablyequal to or less than about 400 Pa·s, equal to or less than about 350Pa·s or less than or equal to about 300 Pa·s. Viscosity was measuredusing a parallel plate viscometer.

Calcined cement compositions described herein can exhibit an elasticmodulus equal to or less than about 1×10⁶ Pa, equal to or less thanabout 7×10⁵ Pa, equal to or less than about 5×10⁵ Pa or equal to or lessthan about 4×10⁵ Pa. In certain embodiments the elastic modulus is in arange from about 2×10⁵ Pa to about 6×10⁵ Pa.

According to another exemplary embodiment the skin composition mayinclude a cement comprising an inorganic filler material having a firstcoefficient of thermal expansion from 25° C. to 600° C. and acrystalline inorganic fibrous material having a second coefficient ofthermal expansion from 25° C. to 600° C.

In exemplary embodiments, the inorganic filler material comprises atleast 10% of the total weight of the inorganic solid components of thecement mixture and the crystalline inorganic fibrous material comprisesless than 25% of the total weight of the inorganic solid components ofthe cement mixture. In exemplary embodiments, the first coefficient ofthermal expansion is less than 50% of the second coefficient of thermalexpansion.

For example, the inorganic filler material can comprise from 20% to 80%,such as from 25% to 75%, and further such as from 30% to 70%, and yetfurther such as from 35% to 65%, including at least 50%, at least 60%,or at least 70% of the total weight of the inorganic solids componentsof the cement mixture. The crystalline inorganic fibrous material cancomprise from 3% to 20%, such as from 5% to 15%, and further such asfrom 8% to 12%, including less than 10%, less than 15%, or less than 20%of the total weight of the inorganic solids components of the cementmixture.

The first coefficient of thermal expansion, can, in certain exemplaryembodiments range from 0.5×10⁻⁷° C. to 20×10⁻⁷/° C., such as from1.0×10⁻⁷° C. to 10×10⁻⁷° C. and further such as from 2.0×10⁻⁷/° C. to5×10⁻⁷/° C., including less than 7×10⁻⁷/° C., and including about2.5×10⁻⁷/° C. The second coefficient of thermal expansion can, incertain exemplary embodiments, range from 10×10⁻⁷° C. to 100×10⁻⁷° C.,such as from 20×10⁻⁷° C. to 90×10⁻⁷/° C., and further such as from30×10⁻⁷/° C. to 80×10⁻⁷/° C., including at least 50×10⁻⁷/° C., andincluding about 65×10⁻⁷/° C. The first coefficient of thermal expansioncan, in certain exemplary embodiments, be less than 5×10⁻⁷° C. while thesecond coefficient of thermal expansion can be greater than 30×10⁻⁷/° C.The first coefficient of thermal expansion can, in certain exemplaryembodiments, be less than 25% of the second coefficient of thermalexpansion, such as less than 20% of the second coefficient of thermalexpansion, and further such as less than 15% of the second coefficientof thermal expansion, and yet further such as less than 10% of thesecond coefficient of thermal expansion, and still yet further such asless than 5% of the second coefficient of thermal expansion, such asfrom 1% to 20% of the second coefficient of thermal expansion, andfurther such as from 2% to 10% of the second coefficient of thermalexpansion.

In certain exemplary embodiments, at least 50% by weight of thecrystalline inorganic fibrous material has an aspect ratio (longestdimension divided by shortest dimension) of from 3:1 to 10:1, such asfrom 4:1 to 8:1. In certain exemplary embodiments, less than 10% byweight of the crystalline inorganic fibrous material has an aspect ratioof less than 3:1. In certain exemplary embodiments, less than 5% byweight of the crystalline inorganic fibrous material has an aspect ratioof less than 3:1. In certain exemplary embodiments, the average aspectratio of the crystalline inorganic fibrous material is from 3:1 to 10:1,such as from 4:1 to 8:1, including about 5:1.

In certain exemplary embodiments, the crystalline inorganic fibrousmaterial has an average diameter of from 2 to 80 microns, such as from 5to 50 microns, and further such as from 10 to 30 microns. Thecrystalline inorganic fibrous material can, in certain exemplaryembodiments, have an average length of from 10 to 500 microns, such asfrom 50 to 400 microns, and further such as from 100 to 300 microns.

In certain exemplary embodiments, the crystalline inorganic fibrousmaterial can comprise a finer fibrous material having an averagediameter of from 2 to 10 microns and an average length of from 10 to 50microns. The crystalline inorganic fibrous material may also comprise arelatively coarser fibrous material having an average diameter of from20 to 60 microns and an average length of from 100 to 300 microns. Thecrystalline inorganic fibrous material may also comprise a fibrousmaterial of intermediate coarseness, having an average diameter of from10 to 20 microns and an average length of from 50 to 100 microns.

The crystalline inorganic fibrous material can, in certain exemplaryembodiments, be present in the cement mixture in a single distribution(e.g., of only one of a finer fibrous material, coarser fibrousmaterial, and fibrous material of intermediate coarseness), a bi-modaldistribution (e.g., of two of a finer fibrous material, coarser fibrousmaterial, and fibrous material of intermediate coarseness), or atri-modal distribution (e.g., of three of a finer fibrous material,coarser fibrous material, and fibrous material of intermediatecoarseness).

A finer fibrous material may correlate to a lower amount of said fibrousmaterial being present in certain exemplary cement mixtures havingsimilar characteristics with respect to at least one property.Accordingly, one set of exemplary embodiments includes a cement mixturecomprising a crystalline inorganic fibrous material, which comprisesfrom 3% to 10% of the total weight of the inorganic solid components ofthe cement mixture, wherein the crystalline inorganic fibrous materialhas an average diameter of from 2 to 10 microns and an average length offrom 10 to 50 microns. Exemplary embodiments also include those having acement mixture comprising a crystalline inorganic fibrous material,which comprises from 5% to 15% of the total weight of the inorganicsolid components of the cement mixture, wherein the crystallineinorganic fibrous material has an average diameter of from 10 to 20microns and an average length of from 50 to 100 microns. Exemplaryembodiments also include those having a cement mixture comprising acrystalline inorganic fibrous material, which comprises from 10% to 20%of the total weight of the inorganic solid components of the cementmixture, wherein the crystalline inorganic fibrous material has anaverage diameter of from 20 to 60 microns and an average length of from100 to 300 microns.

In certain exemplary embodiments, less than 5%, such as less than 2%,and further such as less than 1% by weight of the crystalline inorganicfibrous material has a diameter of greater than 250 microns. In certainexemplary embodiments, the crystalline inorganic fibrous material isessentially free of material having a diameter of greater than 250microns.

In certain exemplary embodiments, less than 5%, such as less than 2%,and further such as less than 1% by weight of the crystalline inorganicfibrous material has a diameter of greater than 200 microns. In certainexemplary embodiments, the crystalline inorganic fibrous material isessentially free of material having a diameter of greater than 200microns.

In certain exemplary embodiments, less than 5%, such as less than 2%,and further such as less than 1% by weight of the crystalline inorganicfibrous material has a diameter of greater than 150 microns. In certainexemplary embodiments, the crystalline inorganic fibrous material isessentially free of material having a diameter of greater than 150microns.

In certain exemplary embodiments, less than 5%, such as less than 2%,and further such as less than 1% by weight of the crystalline inorganicfibrous material has a diameter of greater than 100 microns. In certainexemplary embodiments, the crystalline inorganic fibrous material isessentially free of material having a diameter of greater than 100microns.

In certain exemplary embodiments, less than 5%, such as less than 2%,and further such as less than 1% by weight of the crystalline inorganicfibrous material has a diameter of greater than 50 microns. In certainexemplary embodiments, the crystalline inorganic fibrous material isessentially free of material having a diameter of greater than 50microns.

In certain exemplary embodiments, the crystalline inorganic fibrousmaterial consists essentially of inorganic fibers having an aspect ratioof at least 2:1. In certain exemplary embodiments, the crystallineinorganic fibrous material contains less than 5%, such as less than 2%,and further such as less than 1% by weight of shot or filler material.In certain exemplary embodiments, the crystalline inorganic fibrousmaterial is essentially free of shot or filler material.

In certain exemplary embodiments, at least 95%, such as at least 98%,and further such as at least 99% by weight of the crystalline inorganicfibrous material has an aspect ratio of at least 2:1. In certainexemplary embodiments, substantially all of the crystalline inorganicfibrous material has an aspect ratio of at least 2:1.

In certain exemplary embodiments, the crystalline inorganic fibrousmaterial comprises a naturally occurring crystalline inorganic fibrousmaterial. In certain exemplary embodiments, the crystalline inorganicfibrous material comprises an alkaline earth silicate, such as anaturally occurring alkaline earth silicate. An example of a suitablealkaline earth silicate is wollastonite (CaSiO₃), such as that availableunder the trade name Ultrafibe II, available from NYCO MineralsIncorporated, Willsboro, N.Y., USA.

In certain exemplary embodiments, the inorganic filler materialcomprises at least one of ground cordierite and fused silica glasspowder.

In certain exemplary embodiments, the inorganic filler materialcomprises cordierite, such as ground cordierite.

In certain exemplary embodiments, the inorganic filler materialcomprises glass powder, such as fused silica glass powder.

The glass powder filler material can have a median particle size (D50)between 10 and 20 μm, with, for example, a minimum particle size between7 μm and 75 μm and a maximum particle size between 50 μm and 70 μm.Particle size was determined as a mass-based equivalent sphericaldiameter. The glass powder filler material may comprise, for example,from 60% to 80% by weight of the total inorganic components of thecement. Suitable silica powder filler materials are available, forexample, under the trade name Teco-Sil, available from CE Minerals ofTennessee Electro Minerals Incorporated, Tennessee, USA. All particlesize measurements herein were made with a Microtrac Inc. particle sizeanalyzer, unless otherwise indicated.

In another exemplary embodiment, the skin of the honeycomb structure maycomprise an amorphous glass-based cement, the cement formed from acomposition comprising a first (fine) glass powder as a low thermalexpansion filler material, a second (coarse) glass powder as a lowthermal expansion filler material, a crystalline inorganic fibrousmaterial, a binder and a solvent or vehicle for carrying the solidconstituents of the glass-based cement. In certain exemplaryembodiments, the glasses of both the first glass powder filler materialand the second glass powder filler material are amorphous fused silicahaving particle sizes greater than about 1 micron. The distribution ofglass powder filler material particle size may be multimodal in that adistribution of the glass powder filler material with particle sizesgreater than about 1 micron exhibits multiple modes (local maximums) ofparticle sizes. In one embodiment, the amorphous glass-based cementcomprises a bimodal particle size distribution of amorphous glassparticles with a particle size greater than about 1 micron. The glassbased cement may include a first glass powder filler material wherein amedian (D50) particle size of the first glass powder filler material canbe in a range from about 10 microns to about 50 microns, from about 15microns to about 50 microns, from about 20 microns to about 45 micronsor from about 30 microns to about 45 microns, with a D10 in a range fromabout 1 micron to about 10 microns and D90 in a range from about 25microns to about 125 microns. A median (D50) particle size of the secondglass powder filler material can be in a range from about 150 microns toabout 300 microns, in a range from about 150 microns to about 250microns, in a range from about 170 microns to about 230 microns, in arange from about 180 microns to about 220 microns, with D10 in a rangefrom about 100 microns to about 150 microns, and D90 in a range fromabout 250 microns to about 350 microns. Particle sizes are determined asa mass-based equivalent spherical diameter.

The glass based cement may contain, for example, an amount of the firstglass powder filler material in a range from about 20% to about 60% byweight of the total weight of the inorganic solid components of thecement, in a range from about 25% to about 50% by weight, in a rangefrom about 25% to about 40% by weight, or in a range from about 25% toabout 35% by weight. The glass based cement may contain, for example, anamount of the second glass powder filler material in a range from about10% to about 40% by weight of the total weight of the inorganic solidcomponents of the cement, in a range from about 15% to about 40% byweight, in a range from about 20% to about 35% by weight.

In one embodiment, D50 of the first glass powder filler material may bein a range from about 34 microns to about 40 microns, and a medianparticle size of the second glass powder filler material is in a rangefrom about 190 microns to about 280 microns. In one example, the firstglass powder filler material has a D10 of about 6.0 microns, a D50 ofabout 34.9 microns and a D90 of about 99 microns. In another example,the first glass powder filler material has a D10 of about 6.7 microns, aD50 of about 39.8 microns, and a D90 of about 110.9 microns. In stillanother example, the first glass powder has a D10 of about 2.7 microns,a D50 of about 13.8 microns and a D90 of about 37.8 microns, and as yetanother example, the first glass powder filler material has a D10 ofabout 2.8 microns, a D50 of about 17.2 microns and a D90 of about 47.9microns.

The ratio of the second glass powder filler material to the first glasspowder filler material may be in a range from about 1:4 to about 1:1,such as about 1:3.5 to about 1:1, from about 1:3 to about 1:1, fromabout 1:2.5 to about 1:1, from about 1.2 to about 1:1 or from about1:1.5 to about 1:1. In one exemplary embodiment, the ratio of the secondglass powder filler material to the first glass powder filler materialis about 1:1.

To provide the cement compositions of the present disclosure, theinorganic powders comprising any of the above inorganic powders and anyoptional inorganic additive components can be mixed together with asuitable organic and/or inorganic binder material. The organic bindermaterial may comprise one or more organic materials, such as a celluloseether, methylcellulose, ethylcellulose, polyvinyl alcohol, polyethyleneoxide and the like, or in some embodiments a gum-like material such asActigum®, xanthan gum or latex. For example, A4 Methocel is a suitableorganic binder. Methocel A4 is a water-soluble methyl cellulose polymerbinder available from Dow Chemical. The organic binder material may, forexample, be present in the cement composition in an amount of from 0.1%to 10% by weight, such as from 0.2% to 5%, and further such as from 0.5%to 2%.

A suitable inorganic binder may comprise colloidal silica or aluminacomprising nanometer-scale silica or alumina particles suspended in asuitable liquid, such as water. The inorganic binder material may, forexample, be present in the cement composition in an amount less thanabout of from 2% to 35% of the total weight of inorganic solids presentin the cement, and in some embodiments inorganic binders are present inan amount of from 5% to 30%, and in certain other embodiments in anamount of from 10% to 25%. A suitable colloidal silica binder materialis Ludox HS-40 produced by W.R. Grace. Typical colloidal bindermaterials may comprise approximately 40% by weight solid material as asuspension in a deionized water vehicle.

Typically, the preferred liquid vehicle or solvent for providing aflowable or paste-like consistency has included water, such as deionized(DI) water, although other materials may be used. The liquid vehiclecontent may be present as a super addition in an amount equal to or lessthan about 30% of the total weight of the inorganic components of thecement mixture, such as a range of from about 10% to about 25% of thetotal weight of the inorganic components of the cement mixture. However,the liquid vehicle is typically adjusted to obtain a viscosity suitableto make the cement easy to apply.

In some embodiments, the cement may optionally further contain organicmodifiers, such as adhesion promoters for enhancing adhesion between thecement and the honeycomb body. For example, Michem 4983 has been foundsuitable for this purpose.

In certain exemplary embodiments, the cement mixture sets at atemperature of less than 1000° C., such as a temperature of less than800° C., and further such as a temperature of less than 600° C., and yetfurther such as a temperature of less than 400° C., and still yetfurther such as a temperature of less than 200° C. In certain exemplaryembodiments, the cement mixture is capable of setting at roomtemperature (i.e., at about 23° C.).

In certain exemplary embodiments, the cement mixture has an uncalcinedmodulus of rupture of at least 500 psi, such as at least 550 psi, andfurther such as at least 600 psi, and still further such as at least 650psi, and yet still further such as at least 700 psi, and even yet stillfurther such as at least 750 psi.

In certain exemplary embodiments, the cement mixture has an uncalcinedmodulus of rupture of at least 500 psi while the crystalline inorganicfibrous material comprises less than 20% of the total weight of theinorganic solids components of the cement mixture. In certain exemplaryembodiments, the cement mixture has an uncalcined modulus of rupture offrom 500 to 800 psi while the crystalline inorganic fibrous materialcomprises from 3% to 20% of the total weight of the inorganic solidscomponents of the cement mixture. In certain exemplary embodiments, thecement mixture has an uncalcined modulus of rupture of from 500 to 800psi while the crystalline inorganic fibrous material comprises from 5%to 15% of the total weight of the inorganic solids components of thecement mixture.

Cement compositions described herein can exhibit viscosities well suitedfor forming an external skin over a honeycomb body. For example,compositions according to the embodiments herein can have an infiniteshear viscosity equal to or less than about 12 Pascal-seconds (Pa·s.),equal to or less than about 5 Pa·s., or equal to or less than about 4Pa·s. For a shear rate of 10 s⁻¹, the shear viscosity may, for example,be equal to or less than about 400 Pa·s, equal to or less than about 350Pa·s or less than or equal to about 300 Pa·s. Viscosity was measuredusing a parallel plate viscometer.

Calcining of cement compositions disclosed herein can be conducted in abox furnace with a linear ramp to 600° C. in 3 hours, followed by a holdfor 3 hours at 600° C., then followed by a ramp down to room temperatureover a time period of 3 hours. In commercial use, the ceramic articlewould be wash coated with catalyst followed by a heat treatment toremove organic materials. The ceramic article would also be canned witha mat material that may also require heat treatment to remove organicmaterials. The calcining process simulates service conditionsexperienced by the ceramic article.

Calcined cement compositions described herein can exhibit an elasticmodulus equal to or less than about 1×10⁶ Pa, equal to or less thanabout 7×10⁵ Pa, equal to or less than about 5×10⁵ Pa or equal to or lessthan about 4×10⁵ Pa. In certain embodiments the elastic modulus is in arange from about 2×10⁵ Pa to about 6×10⁵ Pa.

Calcined cement compositions described herein can exhibit an averagethermal shock final pass temperature of at least 600° C. using themethod described as follows. First, an oven is preheated to a firsttemperature and stabilized. Then, a room temperature article (i.e. about23° C.), such as a honeycomb body comprising a ceramic skin, such asembodiments thereof described herein, is plunged into the hot oven for30 minutes. After the 30 minute period, the hot article is removed fromthe oven and air quenched back to room temperature without forcedcooling (e.g. blowing chilled air, etc.). The article is transferredfrom the hot and cold locations on low thermal mass ceramic setters (1″cubes of cordierite cellular ceramic). The article is inspected forcracks in the skin and honeycomb body using non-destructive methodsincluding visual inspection (aided with 10× magnification), transmittedlight and ultrasound pulse-echo. The article is deemed to have failedwhen a crack is detected in the honeycomb body or cement skin. When anarticle survives, the oven is set to a higher temperature and theprocess is repeated. The last temperature passed and 1^(st) temperaturefailed bracket the performance of the article. In the case of the datapresented herein, the 1^(st) temperature is 500° C. and each successivestep is an addition of 50° C. The last temperature survived is reported.No temperatures in excess of 1100° C. are used.

Accordingly, in certain embodiments, the calcined cement skin of theceramic structure exhibited no visible cracks under 10× magnificationafter heating to a temperature of 600° C. for 30 minutes followed byunforced cooling to 23° C. In certain other embodiments, the calcinedcement skin of the ceramic structure exhibited no visible cracks under10× magnification after heating to a temperature of 1000° C. for 30minutes followed by unforced cooling to 23° C.

While the cement mixture compositions disclosed herein have beendescribed for skin applications, it is contemplated that they may beused for other applications relating to honeycomb bodies and ceramicparticulate filters including for plugging at least some of the ends ofthe channels of the honeycomb bodies or for adhering segments ofhoneycomb bodies together.

Table 1 shows the physical properties of two different example skincements that have two differing sets of properties. Example 1 is strongand stiff with low porosity as indicated by a MOR of 511 psi and modulusof elasticity (Emod) of 0.49 Mpsi in the green condition and MOR of 280psi and Emod of 0.53 Mpsi after calcination, with a porosity of 37.8%.Example 2 is weak and flexible with high porosity as indicated by a MORof 455 psi and Emod of 0.39 Mpsi in the green condition and MOR of 54psi and Emod of 0.29 Mpsi after calcination, with a porosity of 48.6%.One distinct difference between the two skin cements is the presence ofLudox® (colloidal silica, inorganic binder) in Example 1, which isabsent in Example 2. The Ludox, increases density, particle packing,cohesion, and adhesion, due to its small particle size, higher surfacearea, high surface charge, and ability to penetrate into small pores,cracks, and features of the cement and body.

TABLE 1 physical properties of two different skin cements As-madeCalcined Skin MOR Emod MOR Emod % Material (psi) (Mpsi) (psi) (Mpsi)Porosity Example 1 511 0.49 280 0.53 37.8 Example 2 455 0.39  54 0.2948.6

Oven thermal shock performance of several lots of ceramic honeycombstructure 300/5 substrates with one of the two skin cement compositionsdescribed in Table 1 applied on each lot of ceramic honeycomb structureshowed that thin Example 1 skin layers performed better than thickExample 1 skin layers and that thick and thin Example 2 samples passedall conditions. The skin layers were either applied thick (standard) orthin. In some cases, the exact same lot of matrix material was used forcomparison. In each case the skinned substrates were plunged into anoven at the temperatures from 450° C. for 30 minutes followed by a rapidextraction/cooling to room temperature and a crack inspection.Crack-free samples were then subjected to the next higher temperature,step-wise in 25° C. steps up to 700° C., until failure was observed inthe form of a crack that penetrated into the matrix. Results showed thata thin skin layer of Example 1 performed better than a thick skin layerof Example 1 and that all Example 2 samples passed all conditions. Whilethe Example 2 skin performs well in this test, it is weaker and lessabrasion resistant and may not be as durable in handing as Example 1.

According to exemplary embodiments of the disclosure, the core skin mayinclude at least one of the compositions mentioned above. For example,the core skin layer may comprise at least one of the strong and stiffwith low porosity compositions, the weak and flexible with high porositycompositions, the single glass powder compositions, the dual glasspowder compositions, the single glass powder with fibrous reinforcingmaterial compositions, the dual glass powder with fibrous reinforcingmaterial compositions, the inorganic filler and crystalline inorganicfibrous material compositions, and the dual glass powder and crystallineinorganic fibrous material compositions.

According to exemplary embodiments of the disclosure, the clad skin mayinclude at least one of the compositions mentioned above. For example,the clad skin layer may comprise at least one of the strong and stiffwith low porosity compositions, the weak and flexible with high porositycompositions, the single glass powder compositions, the dual glasspowder compositions, the single glass powder with fibrous reinforcingmaterial compositions, the dual glass powder with fibrous reinforcingmaterial compositions, the inorganic filler and crystalline inorganicfibrous material compositions, and the dual glass powder and crystallineinorganic fibrous material compositions.

The clad skin can be the same composition as the core skin or the cladskin can be a different composition than the core skin. For example, thecore skin may be the single glass powder with fibrous reinforcingmaterial compositions and the clad skin may be the single glass powderwith fibrous reinforcing material compositions. For example, the coreskin may be the single glass powder with fibrous reinforcing materialcompositions and the clad skin may be the dual glass powdercompositions. For example, the core skin may be a weak and flexible withhigh porosity composition such as Example 2, and the clad skin may be astrong and stiff with low porosity composition such as Example 1.

According to exemplary embodiments, the core skin comprises powderparticles as described above. The core skin thickness may be in a rangeof a few powder particle diameters to several millimeters. For example,in a range of 0.10 mm to 5.0 mm or in a range of greater than 5.0 mm.For example, in a range of 0.1 mm to 3.0 mm, in a range of 0.1 mm to 2.0mm, in a range of 0.1 mm to 1.0 mm, in a range of 0.2 mm to 2.5 mm, in arange of 0.2 mm to 1.5 mm, in a range of 0.3 mm to 3.5 mm, or in a rangeof 0.3 mm to 1.5 mm.

According to exemplary embodiments, the clad skin comprises powderparticles as described above. The clad skin thickness may be in a rangeof a few powder particle diameters to several millimeters. For example,in a range of 0.10 mm to 5.0 mm or in a range of greater than 5.0 mm.For example, in a range of 0.1 mm to 3.0 mm, in a range of 0.1 mm to 2.0mm, in a range of 0.1 mm to 1.0 mm, in a range of 0.1 mm to 0.5 mm, in arange of 0.2 mm to 2.5 mm, in a range of 0.2 mm to 1.5 mm, in a range of0.3 mm to 3.5 mm, or in a range of 0.3 mm to 1.5 mm. The thickness ofthe core skin layer and the clad skin layer may vary due to imprecisionof the applicator equipment, for example, centering of a ceramichoneycomb body for a doctor blade applicator. As described below withreference to FIGS. 10A and 10B, according to an exemplary embodiment ofthe disclosure, the dual skin layer may vary in thickness for desiredapplications. This variable thickness of the dual skin layer may not bepossible in a single skin layer due to cracking of the single layer skincaused by variation in the skin thickness.

According to exemplary embodiments, the clad skin thickness may be in arange of 0.10 mm to 5.0 mm when the core skin thickness is in a range of0.10 mm to 5.0 mm. For example, the clad skin thickness may be in arange of 0.1 mm to 3.0 mm, in a range of 0.2 mm to 2.5 mm, or in a rangeof 0.5 mm to 1.5 mm when the core skin thickness may be in a range of0.1 mm to 4.0 mm, in a range of 0.2 mm to 3.5 mm, or in a range of 0.7mm to 2.5 mm. The clad skin thickness may be in a range of 0.1 mm to 1.0mm when the core skin thickness is in a range of 0.1 mm to 3.0 mm. Forexample, the clad skin thickness may be in a range of 0.1 mm to 0.9 mm,in a range of 0.2 mm to 0.7 mm, or in a range of 0.3 mm to 0.5 mm whenthe core skin thickness may be in a range of 0.1 mm to 3.5 mm, in arange of 0.2 mm to 3.0 mm, or in a range of 0.7 mm to 2.5 mm. The totalthickness of the dual skin layer may be in a range from 0.2 mm to 10.0mm, for example, about 0.2 mm to 5.0 mm, about 0.2 mm to 4.0 mm, about0.2 mm to 2.0 mm, about 0.4 mm to 5.0 mm, about 0.4 mm to 4.0 mm, about0.4 mm to 2.0 mm, about 0.9 mm to 5.0 mm, about 0.9 mm to 3.0 mm orabout 0.9 mm to 2.5 mm. According to exemplary embodiments, a thick corecement layer may be applied and dried rapidly to form a thick core skinlayer and a thin clad cement layer may be applied to the thick corelayer and dried more slowly to form a thin clad skin layer and a crackfree dual skin layer.

According to an exemplary embodiment of the disclosure, the core layermay be thicker than the clad layer and have a CTE close to the CTE ofthe ceramic honeycomb body for acceptable in service performance. Theclad layer may have less porosity than the core layer and a higher MORand Emod than the core layer for acceptable handling performance withoutchipping.

In an exemplary embodiment the clad skin layer can be directly on thecore skin layer or an intervening skin layer may be present. Theintervening skin layer may be one or more layers. Each intervening skinlayer may be the same composition or a different composition as the cladand core skin layers. The intervening layer may be a skin layer with aproperty such as hardness, strength, flexibility, CTE, porosity, orparticle size, and the like, intermediate between the core skin layerand the clad skin layer properties. That is, for example, when the coreskin layer has a relatively high porosity and the clad skin layer has arelatively low porosity, the intervening skin layer may have a porositybetween the relatively high porosity of the core skin layer and therelatively low porosity of the clad skin layer. Likewise when there aremore than one intervening layers, each intervening layer may be a skinlayer with a property intermediate between the property of the skinlayer below it and the skin layer above it. In another exemplaryembodiment properties of the intervening skin layers may alternate, forexample, hard/soft/hard/soft, between the core skin layer and the cladskin layer.

In an exemplary embodiment a method of applying a dual layer skin isdisclosed. A honeycomb structure fired and contoured to a diameter andshape may be applied with the dual layer skin, a honeycomb structureincluding a co-extruded skin layer may be applied with the dual layerskin, or a honeycomb structure including honeycomb segments may beapplied with the dual layer skin. FIG. 6 is a flow diagram illustratingan exemplary process to make a double-skinned product according toexemplary embodiments of the disclosure.

In an exemplary embodiment of a method of applying a dual layer skin ona ceramic honeycomb body as shown in FIG. 6, a honeycomb body isprovided in operation 600. A first coat of a cement mixture is appliedto the ceramic honeycomb body in operation 610. The first coat of cementmixture is rapidly dried to form a first skin layer in operation 620.The first coat of cement mixture may be rapidly dried to form a firstskin layer in operation 622 by hot air dryer (high temperature and lowhumidity), in operation 624 by a microwave dryer, in operation 626 by IRdryer or other similar humidity controlled drier (not shown). A secondcoat of a cement mixture is applied in operation 630, and dried gentlyin operation 640 to form a second skin layer on the first skin layer. Inan exemplary embodiment of the method of applying a dual layer skin thefirst coat cement mixture is applied and rapidly dried. The thickness ofthe applied first coat of cement mixture may be in a range between 0.1mm and 5 mm. The first coat cement mixture composition may include atleast one of a strong and stiff with low porosity composition, a weakand flexible with high porosity composition, a single glass powdercomposition, a dual glass powder composition, a single glass powder withfibrous reinforcing material composition, a dual glass powder withfibrous reinforcing material composition, an inorganic filler andcrystalline inorganic fibrous material composition, and a dual glasspowder and crystalline inorganic fibrous material composition. The firstcoat cement mixture may be applied by an axial application method, adoctor blade squeegee application method, a spray casting method, a tapecasting method, a spin coating method, and the like.

The first coat cement mixture may be rapidly dried in a process thatuses high temperature hot air, forced air, microwave, RF, IR radiation,or the like. For example, the first coat cement layer may be dried underhigh temperature forced air at a temperature in a range of 25 to 150° C.in a time range of 30 s to 3 h. For example, the first coat cement layermay be dried under high temperature forced air at a temperature in arange of 25 to 150° C. in a time range of less than 120 min, for exampleless than 90 min, less than 60 min or even less than 30 min. Forexample, the first coat cement layer may be dried in a microwave at 10kW to 200 kW with forced air at a temperature in a range of 100 to 150°C. in a time range of less than 1 min.

In an exemplary embodiment of the method of applying a dual layer skinthe second coat of a cement mixture is applied to the first skin layerand gently dried. The thickness of the applied second coat cementmixture may be in a range between 0.1 mm and 5 mm. The second coatcement mixture composition may include at least one of a strong andstiff with low porosity composition, a weak and flexible with highporosity composition, a single glass powder composition, a dual glasspowder composition, a single glass powder with fibrous reinforcingmaterial composition, a dual glass powder with fibrous reinforcingmaterial composition, an inorganic filler and crystalline inorganicfibrous material composition, and a dual glass powder and crystallineinorganic fibrous material composition. The second coat cement mixturemay be applied by an axial application method, a doctor blade squeegeeapplication method, a spray casting method, a tape casting method, aspin coating method, and the like.

The second coat cement mixture may be gently dried in a process thatuses high temperature hot air, forced air, microwave, RF, IR radiation,or the like. For example, the second coat cement layer may be driedunder high temperature forced air at a temperature in a range of 100 to150° F. in a time range of 30 s to 2 h. For example, the second coatcement layer may be dried under high temperature forced air at atemperature in a range of 100 to 150° F. in a time range of 1 min to 5min. For example, the second coat cement layer may be dried in amicrowave at 10 kW to 200 kW with forced air at a temperature in a rangeof 100 to 150° F. in a time range of less than 1 min.

According to exemplary embodiments of the disclosure, a crack free dualskin layer may be achieved that has a thickness comparable to typicalsingle layer skin in less time, because of rapid drying of the corelayer followed by gentle drying of the clad layer. Although the cladlayer is dried gently (lower heat and longer times than the rapiddrying), comparable to a single skin layer, the time period for dryingis shorter because the clad layer is the outer layer of the dual skinlayer. Therefore the clad layer is thinner than the typical single layerskin.

The method for applying the first coat cement mixture and the secondcoat cement mixture may be the same or may be different. For example,the first coat cement mixture may be applied by a doctor blade methodand the second coat cement mixture may be applied by an axialapplication method. For example, the first coat cement mixture may beapplied by a doctor blade method and the second coat cement mixture maybe applied by a doctor blade application method. For example, the firstcoat cement mixture may be applied by an axial application method andthe second coat cement mixture may be applied by an axial applicationmethod.

Furthermore, intervening coats of cement mixture may be applied on thefirst coat cement layer before the second coat cement layer is applied.In exemplary embodiments where an intervening layer is applied to thecore layer, the intervening layer may be applied as described in detailfor the first and second coat cement layers. Therefore, further detaileddescription will be omitted here.

According to an exemplary embodiment of the disclosure, a system forapplying the dual layer skin is provided. FIG. 7 is a schematic of ahoneycomb body multilayer skinning system 700 according to an exemplaryembodiment of the disclosure. The system comprises a first skinning unit710, a first drier unit 720, a second skinning unit 730, and a seconddrier unit 740. The first skinning unit 710 may be an axial skinner, adoctor blade skinner, a spray cast skinner, a tape cast skinner, a spincoat skinner and the like. The first skinning unit 710 receives thehoneycomb body 12 to be skinned at operation 712 and coats the honeycombbody 12 with a first cement layer. The first drier unit 720 may be a hotair drier, a forced air drier, a microwave drier, an RF drier, an IRradiation drier, and the like. The first drier unit 720 receives thehoneycomb body 12 having the first cement layer at operation 722 anddries the first cement layer to form a first skin layer 36 as shown inFIGS. 1 and 2.

The second skinning unit 730 may be an axial skinner, a doctor bladeskinner, a spray cast skinner, a tape cast skinner, a spin coater, andthe like. The second skinning unit 730 receives the honeycomb body 12having the first skin layer 36 at operation 732 and coats the first skinlayer 36 with a second cement layer. The second drier unit 740 may be ahot air drier, a forced air drier, a microwave drier, an RF drier, an IRradiation drier, and the like. The second drier unit 740 receives thehoneycomb body 12 having the second cement layer at operation 742 anddries the second cement layer to form a second skin layer 40 as shown inFIGS. 1 and 2 at operation 754. The first skin layer 36 may be the corelayer and the second skin layer 40 may be the clad layer. Optionally,the system may include additional coating and drying units forintervening skin layers 44. An exemplary embodiment of an interveningskin layer 44 is shown in FIG. 8.

According to an exemplary embodiment the system may omit the seconddrier unit 740 by process 734 and dry the second cement layer in thefirst drier unit 720. Optionally, the system may omit the secondskinning unit 730 and the second drier unit 740 by process 744. In suchan exemplary embodiment, the first skinning unit 710 receives thehoneycomb body 12 having the first skin layer 36 at operation 744 andcoats the first skin layer 36 with a second cement layer. The firstdrier unit 720 receives the honeycomb body 12 having the second cementlayer at operation 722 and dries the second cement layer to form asecond skin layer 40. Furthermore, intervening skin layers 44 may beapplied by operations 744 and 722 before proceeding to operation 732 orby operations 734 and 732 before operation 742.

FIG. 9A is a photograph showing an example core skin on a 2 in diameterhoneycomb body according to an exemplary embodiment. Defects resultingfrom fast drying are visible. FIG. 9B is a photograph showing an exampleclad skin on a 2 in diameter honeycomb body having a core skin as shownin FIG. 9A according to an exemplary embodiment. Defects are eliminatedthrough the second layer of skin. FIG. 9C is a photograph of thecorresponding ends of the examples shown in FIGS. 9A and 9B.

FIG. 10A is a photograph showing an example core skin on a 2 in diameterhoneycomb body according to an exemplary embodiment. Defects resultingfrom fast drying are visible. FIG. 10B is a photograph showing anexample clad skin on a 2 in diameter honeycomb body having a core skinas shown in FIG. 10A according to an exemplary embodiment. Defects areeliminated through the second layer of skin. FIG. 10C is a photograph ofthe corresponding end of the example shown in FIG. 10B.

FIG. 11A is a photograph showing a side view of an example dual layerskin on a 2 in diameter honeycomb body according to an exemplaryembodiment. FIG. 11B is a photograph showing an end view of the duallayer skin on the 2 in diameter honeycomb body shown in FIG. 11A. Thedual layer skin can be crack-free even when off-center according toexemplary embodiments as demonstrated by the example shown in FIGS. 11Aand 11B.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of making a honeycomb structure,comprising: coating the periphery of a honeycomb body with a firstcement mixture to form a first coat cement mixture; drying the firstcoat cement mixture to form a first skin layer; coating the first skinlayer with a second cement mixture to form a second coat cement mixture;and drying the second coat cement mixture on the periphery of thehoneycomb body to form a second skin layer and the honeycomb structure,wherein the first skin layer comprises a first porosity, a first modulusof rupture, and a first modulus of elasticity, and the second skin layercomprises a second porosity less than the first porosity, a secondmodulus of rupture greater than the first modulus of rupture, and asecond modulus of elasticity greater than the first modulus ofelasticity.
 2. The method according to claim 1, wherein drying the firstcoat cement mixture comprises exposing the first coat cement mixture toat least one of high temperature air, forced air, microwave, RF, and IRradiation, and drying the second cement mixture comprises exposing thesecond coat cement mixture to at least one of high temperature air,forced air microwave, RF, and IR radiation.
 3. The method according toclaim 1, wherein drying the first coat cement mixture comprises exposingthe first coat cement mixture to high temperature forced air in atemperature range of 25 to 200° C., and drying the second cement mixturecomprises exposing the second coat cement mixture to high temperatureair in a temperature range of 25 to 140° C.
 4. The method according toclaim 3, wherein drying the first coat cement mixture comprises exposingthe first coat cement mixture to high temperature forced air in atemperature range of 100 to 200° C. for less than 90 min, and drying thesecond cement mixture comprises exposing the second coat cement mixtureto high temperature air in a temperature range of 25 to 60° C. for lessthan 60 min.
 5. The method according to claim 1, wherein coating theperiphery of the honeycomb body with a first cement mixture comprisesapplying the first cement mixture at a thickness of 0.10 mm to 5.0 mm,and coating the first skin layer with a second cement mixture comprisesapplying the second cement mixture at a thickness of 0.10 mm to 5.0 mm.6. The method according to claim 5, wherein coating the periphery of thehoneycomb body with a first cement mixture comprises applying the firstcement mixture at a thickness of 0.10 mm to 3.00 mm, and coating thefirst skin layer with a second cement mixture comprises applying thesecond cement mixture at a thickness of 0.10 mm to 1.00 mm.
 7. Themethod according to claim 1, wherein coating the periphery of thehoneycomb body comprises at least one of an axial application of thefirst cement mixture, a doctor blade application of the first cementmixture, a spray casting of the first cement mixture, a tape casting ofthe first cement mixture, a spin coating of the first cement mixture,and coating the first skin layer with the second cement mixturecomprises at least one of an axial application of the second cementmixture, a doctor blade application of the second cement mixture, aspray casting of the second cement mixture, a tape casting of the secondcement mixture, and a spin coating of the second cement mixture.
 8. Themethod according to claim 1, wherein a coefficient of thermal expansionof the first skin layer is substantially equal to a thermal expansioncoefficient of the honeycomb body, and a coefficient of thermalexpansion of the second skin layer is greater than the coefficient ofthermal expansion of the first skin layer.